An IR-FPA includes an array of radiation detectors that views a scene of interest, detects thermal radiation arriving from the scene, and provides an image of the scene. A given radiation detector may be referred to as a unit cell or pixel. Incident radiation arriving from the scene of interest is converted by the radiation detectors to electric charge and integrated within each unit cell. The integrated charge within each unit cell of the IR-FPA is readout to form an analog output signal of the IR-FPA. A system analog-to-digital converter (ADC) converts the analog output signal of the IR-FPA into a digital signal. The digital signal is processed to provide the image of the scene of interest.
Referring to FIG. 1, a typical embodiment of an N row by M column IR-FPA is shown. In FIG. 1, each column of unit cells is provided with a column amplifier, such as a capacitive transimpedance amplifier or CTIA. A first set of transistors (connected to a potential (V.sub.IN)) are controlled to properly bias a unit cell photodetector (PD), while a second set of transistors which function as switches (connected to timing signals transfer-phase or .PHI.tr) are used to sequentially and individually connect N unit cells along a given column to a column sense or readout line 1 that is connected to the column's column amplifier (the CTIA). Each of the M column CTIAs may readout all the unit cells in parallel across a given one of the N rows, and then output a signal having a magnitude that is indicative of the radiation that was detected by the unit cells. This process continues until all N rows have been readout, thus forming an image frame. During an integration period, before readout, the detected current (I.sub.DET) is integrated on a unit cell capacitance (C.sub.INT). For a CTIA embodiment the column amplifier includes an integration capacitance and a solid state switch for periodically resetting the integration capacitance.
In an ideal case, each of the detector elements of the IR-FPA would operate in an identical manner so that each detector element would produce the same output signal in response to a particular input from the scene of interest. Realistically, uniformity in all detector elements is difficult to achieve, particularly when the N.times.M array is a large array. In view of this, techniques have been developed to suppress FPA non-uniformities. For example, commonly assigned U.S. Pat. No. 4,975,864, issued Dec. 4, 1990, entitled "Scene Based Nonuniformity Compensation for Staring Focal Plane Arrays", by Sendall et al., discloses scene-based nonuniformity compensation that serves to reduce stationary, as contrasted to moving, nonuniformities in the FPA's output signal. Similarly, commonly assigned U.S. Pat. No. 5,323,334, issued Jun. 21, 1994, entitled "Sensor System Having Nonuniformity Suppression with Image Preservation", by Meyers et al., discloses suppression of detector-based nonuniformities, without the loss of stationary features of the viewed image, by providing a physically moveable sensor which progressively corrects pixel intensities for detector element non-uniformity. The disclosure of these U.S. Patents are incorporated by reference herein in their entirety.
Currently available so-called second-generation IR-FPAs are capable of providing high dynamic range levels. High dynamic range levels have a potential for improving the image quality of the scene of interest. However, because the system ADC is generally limited in resolution and in conversion speed by power and space constraints, the system ADC may be unable to capture the full instantaneous dynamic range of the IR-FPA. Thus, image quality may be degraded. Additionally, non-uniformities in the IR-FPA output signal degrades image quality. The non-uniformities limit the system ADC's ability to capture the full instantaneous dynamic range of the IR-FPA. In view of this, techniques have been developed to adaptively control dynamic range and thus reduce the resolution required for the system ADC. For example, commonly assigned U.S. Pat. No. 5,563,405, issued Oct. 8, 1996, entitled "Staring IR-FPA With Adaptive Dynamic Range Control Electronics", by Woolaway, II et al., discloses a circuit architecture which provides an adaptive feedback to enable charge pedestal suppression on a per-pixel basis and thus achieve a higher dynamic range. The disclosure of this U.S. Patent is incorporated by reference herein in its entirety.
In addition to the non-uniformities discussed above, non-uniformities in the IR-FPA output signal may be introduced by such factors as system drift and 1/f noise resulting in spatial fixed pattern noise. In the prior art, IR-FPAs have employed readout integrated circuits (ROICs) having subframe averaging circuits to improve signal-to-noise ratios. For example, U.S. Pat. No. 4,779,004, issued Oct. 18, 1988, entitled "Infrared Imager", by Tew et al., discloses an IR-FPA employing a ROIC having an averaging circuit for recursive integration to enhance the signal-to-noise ratio of the IR-FPA. In particular, Tew et al. teaches an integration mode in which charge residing on an input capacitor is averaged X times per frame onto an averaging capacitor at each pixel of the array. In each frame, after the X times per frame, the charge residing on the averaging capacitors in each pixel is readout and the averaging capacitors are reset.
It can be appreciated that it would be desirable to enhance the non-uniformity correction or signal-to-noise ratio of the ROIC and thus, reduce the spatial or temporal noise of the ROIC, respectively. The prior art discussed above does not adequately address these needs.